Scientists using different methods to determine the mass of galaxies have found a discrepancy that suggests ninety percent of the universe is matter in a form that cannot be seen. Some scientists think dark matter is in the form of massive objects, such as black holes, that hang out around galaxies unseen. Other scientists believe dark matter to be subatomic particles that rarely interact with ordinary matter. This paper is a review of current literature. I look at how scientists have determined the mass discrepancy, what they think dark matter is and how they are looking for it, and how dark matter fits into current theories about the origin and the fate of the universe.
In 1933, the astronomer Fritz Zwicky was studying the motions of distant galaxies. Zwicky estimated the total mass of a group of galaxies by measuring their brightness. When he used a different method to compute the mass of the same cluster of galaxies, he came up with a number that was 400 times his original estimate (1). This discrepancy in the observed and computed masses is now known as "the missing mass problem." Nobody did much with Zwicky's finding until the 1970's, when scientists began to realize that only large amounts of hidden mass could explain many of their observations (2). Scientists also realize that the existence of some unseen mass would also support theories regarding the structure of the universe (3). Today, scientists are searching for the mysterious dark matter not only to explain the gravitational motions of galaxies, but also to validate current theories about the origin and the fate of the universe.
Mass and Weight. What exactly is mass? Most people would say that mass is what you weigh. But to scientists, mass and weight are different things. Mass is the measure of a quantity of matter--how much stuff there is. Weight, on the other hand, is the effect that gravity has on that stuff. Weight is dependent on mass--the more mass you have, the more gravity pulls you down, and the more you weigh. When an astronaut floats in space, we say that the astronaut is weightless. But the astronaut still has a body, and so has mass.
Hide and Seek. Scientists estimate that 90 to 99 percent of the total mass of the universe is missing matter (4). Actually, "missing matter" may be misleading--it's really the light that is missing (5). Scientists can tell that the dark matter is there, but they cannot see it. Bruce H. Margon, chairman of the astronomy department at the University of Washington, told the New York Times, "It's a fairly embarrassing situation to admit that we can't find 90 percent of the universe" (6). This problem has scientists scrambling to try and find where and what this dark matter is. "What it is, is any body's guess," adds Dr. Margon. "Mother Nature is having a double laugh. She's hidden most of the matter in the universe, and hidden it in a form that can't be seen" (5).
Determining the Mass of Galaxies
How do we measure the mass of the universe? Since the boundaries (if there are any) of the universe are unknown, the actual mass of the universe is also unknown. But scientists talk of the missing mass of the universe in percentages, not real numbers. Since the majority of the matter that we can see is clumped together into galaxies, the total mass of all the galaxies should be a good indication of the mass of the universe. Although it isn't possible to add up an infinite number of galaxies, scientists can infer the percentage of the universe's missing mass from estimates of the missing mass in galaxies and clusters of galaxies (7). And because scientists (like Fritz Zwicky) use different techniques to determine the masses of galaxies, they can perceive mass that they cannot see.
The Doppler Shift. One of the tools that scientists use to detect the motions of galaxies is the Doppler Shift. The Doppler Shift was discovered in the 1800's by Christian Doppler when he noticed that sound travels in waves much like waves on the surface of the ocean (7). Doppler also noticed that when the source of the sound is moving, the pitch of the sound is different, depending on whether the source is moving toward or away from the observer. Take, for example, the horn on a train. As the speeding train passes by you, the sound of the horn changes to a lower pitch. This is the Doppler Shift. When the train approaches, the sound waves get pushed together by the motion of the train. As the train speeds away, the sound waves get stretched out.
The Doppler Shift also works with light. When a light source is moving toward you, the light becomes bluer (called a blue shift). When a light source is moving away from you, the light becomes redder (called a red shift). And the faster something is moving, the farther the light is shifted. But the Doppler shift for light is very subtle and cannot be detected with the naked eye. Scientists use a device called a spectroscope to measure Doppler Shift and determine how fast stars and galaxies are moving (7).
Rotational Velocity. Using the power of the Doppler Shift, scientists can learn much about the motions of galaxies. They know that galaxies rotate because, when viewed edge-on, the light from one side of the galaxy is blue shifted and the light from the other side is red shifted. One side is moving toward the Earth, the other is moving away. They can also determine the speed at which the galaxy is rotating from how far the light is shifted (7). Knowing how fast the galaxy is rotating, they can then figure out the mass of the galaxy mathematically.
As scientists look closer at the speeds of galactic rotation, they find something strange. The individual stars in a galaxy should act like the planets in our solar system--the farther away from the center, the slower they should move. But the Doppler Shift reveals that the stars in many galaxies do not slow down at farther distances. And on top of that, the stars move at speeds that should rip the galaxy apart; there is not enough measured mass to supply the gravity needed to hold the galaxy together (7).
These high rotational speeds suggest that the galaxy contains more mass than was calculated. Scientists theorize that, if the galaxy was surrounded by a halo of unseen matter, the galaxy could remain stable at such high rotational speeds.
Seeing the Light. Another method astronomers use to determine the mass of a galaxy (or cluster of galaxies) is simply to look at how much light there is. By measuring the amount of light reaching the earth, the scientists can estimate the number of stars in the galaxy. Knowing the number of stars in the galaxy, the scientists can then mathematically determine the mass of the galaxy(1).
Fritz Zwicky used both methods described here to determine the mass of the Coma cluster of galaxies over half a century ago. When he compared his data, he brought to light the missing mass problem. The high rotational speeds that suggest a halo reinforce Zwicky's findings. The data suggest that less than 10% of what we call the universe is in a form that we can see (8). Now scientists are diligently searching for the elusive dark matter--the other 90% of the universe.
What do scientists look for when they search for dark matter? We cannot see or touch it: its existence is implied. Possibilities for dark matter range from tiny subatomic particles weighing 100,000 times less than an electron to black holes with masses millions of times that of the sun (9). The two main categories that scientists consider as possible candidates for dark matter have been dubbed MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles). Although these acronyms are amusing, they can help you remember which is which. MACHOs are the big, strong dark matter objects ranging in size from small stars to super massive black holes (1). MACHOs are made of 'ordinary' matter, which is called baryonic matter. WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs.
Astronomers and particle physicists disagree about what they think dark matter is. Walter Stockwell, of the dark matter team at the Center for Particle Astrophysics at U.C. Berkeley, describes this difference. "The nature of what we find to be the dark matter will have a great effect on particle physics and astronomy. The controversy starts when people made theories of what this matter could be--and the first split is between ordinary baryonic matter and non-baryonic matter" (10). Since MACHOs are too far away and WIMPs are too small to be seen, astronomers and particle physicists have devised ways of trying to infer their existence.
Massive Compact Halo Objects are non-luminous objects that make up the halos around galaxies. Machos are thought to be primarily brown dwarf stars and black holes (2). Like many astronomical objects, their existence had been predicted by theory long before there was any proof. The existence of brown dwarfs was predicted by theories that describe star formation (7). Black holes were predicted by Albert Einstein's General Theory of Relativity (11).
Brown Dwarfs. Brown dwarfs are made out of hydrogen--the same as our sun but they are typically much smaller. Stars like our sun form when a mass of hydrogen collapses under its own gravity and the intense pressure initiates a nuclear reaction, emitting light and energy. Brown dwarfs are different from normal stars. Because of their relatively low mass, brown dwarfs do not have enough gravity to ignite when they form (7). Thus, a brown dwarf is not a "real" star; it is an accumulation of hydrogen gas held together by gravity. Brown dwarfs give off some heat and a small amount of light (7).
Black Holes. Black holes, unlike brown dwarfs, have an over-abundance of matter. All that matter "collapses" under its own enormous gravity into a relatively small area. The black hole is so dense that anything that comes too close to it, even light, cannot escape the pull of its gravitational field (11). Stars at safe distance will circle around the black hole, much like the motion of the planets around the sun (7). Black holes emit no light; they are truly black.
Astronomers are faced with quite a challenge with detecting MACHOs. They must detect, over astronomical distances, things that give off little or no light. But the task is becoming easier as astronomers create more refined telescopes and techniques for detecting MACHOs.
Searching with Hubble. With the repair of the Hubble Space Telescope, astronomers can detect brown dwarfs in the halos of our own and nearby galaxies. Images produced by the Hubble Telescope, however, do not reveal the large numbers of brown dwarfs that astronomers hoped to find. "We expected [the Hubble images] to be covered wall to wall by faint, red stars," reported Francesco Paresce of the Johns Hopkins University Space Telescope Science Institute in the Chronicle of Higher Education (5). Research results are disappointing--calculations based on the Hubble research estimate that brown dwarfs constitute only 6% of galactic halo matter (12).
Gravitational Lensing. Astronomers use a technique called gravitational lensing in the search for dark matter halo objects. Gravitational lensing occurs when a brown dwarf or a black hole passes between a light source, such as a star or a galaxy, and an observer on the Earth. The object focuses the light rays, causing the light source to brighten (13). Astronomers diligently search photographs of the night sky for the telltale brightening that indicates the presence of a MACHO.
Wouldn't a MACHO block the light? How can dark matter act like a lens? The answer is gravity. Albert Einstein proved in 1919 that gravity bends light rays (13). He predicted that a star, which was positioned behind the sun, would be visible during a total eclipse. Einstein was right--the gravity of the sun bent the light rays coming from the star and made it appear next to the sun.
Not only can astronomers detect MACHOs with the gravitational lens technique, but they can also calculate the mass of the MACHO by determining distances and the duration of the lens effect (13). Although gravitational lensing has been known since Einstein's demonstration, astronomers have only begun to use the technique to look for MACHOs in the past two or three years.
Gravitational Lensing projects include the MACHO project (America and Australia), the EROS project (France), and the OGLE project (America and Poland). Preliminary data from these projects suggest the existence of lens objects with masses between that of Jupiter and the sun (9).
Circling Stars. Another way to detect a black hole is to notice the gravitational effect that it has on objects around it. When astronomers see stars circling around something, but cannot see what that something is, they suspect a black hole. And by observing the circling objects, the astronomers can conclude that, indeed, a black hole does exist.
In January of 1995, a team of American and Japanese scientists announced "compelling evidence" for the existence of a massive black hole at the American Astronomical Society meeting (14). Led by Dr. Makoto Miyosi of the Mizusawa Astrogeodynamics Observatory and Dr. James Moran of the Harvard-Smithsonian Center for Astrophysics, this group calculated the rotational velocity from the Doppler shifts of circling stars to determine the mass of the black hole. This black hole has a mass equivalent to 36 million of our suns (15). While this finding and others like it are encouraging, MACHO researchers have not turned up enough brown dwarfs and black holes to account for the missing mass. Thus, most scientists concede that dark matter is a combination of baryonic MACHOs and non-baryonic WIMPs.
In their efforts to find the missing 90% of the universe, particle physicists theorize the existence of tiny non-baryonic particles that are different from what we call "ordinary" matter. Smaller than atoms, Weakly Interactive Massive Particles are thought to have mass, but usually interact with baryonic matter gravitationally--they pass right through ordinary matter. Since each WIMP has only a small amount of mass, there needs to be a large number of them to make up the bulk of the missing matter. That means that millions of WIMPs are passing through ordinary matter--the Earth and you and me--every few seconds (8). Although some people claim that WIMPs were proposed only because they provide a "quick fix" to the missing matter problem, most physicists believe that WIMPs do exist (4). According to Walter Stockwell, astronomers also concede that at least some of the missing matter must be WIMPs. "I think the MACHO groups themselves would tell you that they can't say MACHOs make up the dark matter" (10). The problem with searching for WIMPs is that they rarely interact with ordinary matter, which makes them difficult to detect.
Detecting WIMPs. All hope of proving WIMPs exist rest on the theory that, on occasion, a WIMP will interact with ordinary matter. Because WIMPs can pass through ordinary matter, a rare WIMP interaction can take place inside a solid object. The trick to detecting a WIMP is to witness one of these interactions. Dr. Bernard Sadoulet and Walter Stockwell at the Center for Particle Astrophysics hope to do just that. Their project involves cooling a large crystal to almost absolute zero, which restricts the motions of its atoms. The energy created by a WIMP interaction with an atom in the crystal will then register on their instruments as heat (8). Because their research is still in progress, there are no results available.
A similar WIMP detection project is under way in Antarctica. The AMANDA project (Antarctica Muon and Neutrino Detector Array) is a collaboration of the University of Chicago, Princeton University, and AT&T, which is partially funded by the National Science Foundation. AMANDA scientists are placing detection instruments deep within the Antarctic ice. Instead of using a crystal, like the Berkeley team, the AMANDA group is using the Antarctic ice sheet itself as a WIMP detector (16).
Dark Matter and the Universe
The search for dark matter is about more than explaining discrepancies in galactic mass calculations. The missing matter problem has people questioning the validity of current theories about how the universe formed, and how it will ultimately end.
The Big Bang. In the mid 1950's a new theory of how the universe formed emerged. The Big Bang theory says that the universe began with a great explosion. The theory evolved from Doppler shift observations of galaxies (17). It seems that, no matter which direction astronomers point their telescopes, the light from the center of the galaxies is red shifted. (Doppler shift caused by rotational velocity can only be detected at the sides of a galaxy.) Observing red-shifted galaxies in every direction implies expansion in all directions an expanding universe.
The Big Bang theory is a current model for the origin of our universe which says all the matter that exists was, at one time, compressed into a single point. The Big Bang distributed all the matter evenly in all directions. Then the matter started to clump together, attracted by gravity, to form the stars and galaxies that we see today. The expansion generated by the Big Bang was great enough to overcome gravity. We still see the effects of that force when we see red-shifted galaxies.
Clumping. One of the problems with the Big Bang theory is its failure to explain how stars and galaxies could form in a young universe that was evenly distributed in all directions. What started the clumping? In a smooth universe, every particle would have the same gravitational effect on every other particle; the universe would remain the same (6). But something supplied the initial gravity to allow galaxies to form. Physicists suggest dark matter WIMPs as the solution. Since WIMPs only affect baryon matter gravitationally, physicists say this dark matter could be the "seed" of galactic formation (6). "We don't have a completely successful model of galaxy formation," explains Walter Stockwell, "but the most successful models to date seem to need plenty of non-baryonic dark matter" (10).
Closed, Open and Flat. There are three current scenarios that predict the future of the universe (17). If the universe is closed, gravity will catch up with the expansion and the universe will eventually be pulled back into a single point. This model suggests an endless series on Big Bangs and "Big Crunches." An open universe has more bang than gravity--it will keep expanding forever. And the flat universe has exactly enough mass to gravitationally stop the universe from expanding, but not enough to pull itself back in. A flat universe is said to have a critical density of 1.
What does the expansion of the universe have to do with the missing mass? The more mass, the more gravity. Whether the universe is closed, open, or flat depends on how much mass there is. This is where dark matter comes into the picture. Without dark matter, critical density lies somewhere between 0.1 and 0.01, and we live in an open universe. If there is a whole lot of dark matter, we could live in a closed universe. Just the right amount of dark matter, and we live in a flat universe. The amount of dark matter that exists determines the fate of the universe.
Many Theories. Scientists are tossing theories back and forth. Some are skeptical of WIMPs; particle physicists say MACHOs will never account for 90% of the universe. Some, like H.C. Arp, G. Burbage, F. Hoyle, and J.V. Narlikan claim that discrepancies like the dark matter problem discredits the Big Bang theory. In Nature they proclaim, "We do not believe that it is possible to advance science profitably when the gap between theoretical speculation becomes too wide, as we feel it has . . . over the past two decades. The time has surely come to open doors, not to seek to close them by attaching words like 'standard' and 'mature' to theories that, judged from their continuing non-performance, are inadequate" (18). Others say there is no missing mass. In his book, What Matters: No Expanding Universe No Big Bang, J.L. Riley claims that galactic red shift is just the effect of light turning into matter as it ages, and not the universe expanding (19).
But most scientists like Walter Stockwell have faith in the Big Bang. "The theorists will come up with all sorts of reasons why this or that can or cannot be and change their minds every other year," he says. "We experimentalists will trudge ahead with our experiments. The Big Bang theory will outlive any of this stuff. It works very well as the overall framework to explain how the universe is today" (10).
Now the missing mass problem is threatening humankind's place in the universe again. If non-baryonic dark matter does exist, then our world and the people in it will be removed even farther from the center. Dr. Sadoulet tells the New York Times, "It will be the ultimate Copernican revolution. Not only are we not at the center of the universe as we know it, but we aren't even made up of the same stuff as most of the universe. We are just this small excess, an insignificant phenomenon, and the universe is something completely different" (20).
A dark matter discovery could possibly affect our view of our place in the universe. If scientists prove that non-baryonic matter does exist, it would mean that our world and the people in it are made of something which comprises an insignificant portion of the physical universe. A discovery of this nature, however, probably will not affect our day-to-day process of living. "It's hard for me to imagine people getting bothered by the fact that most of the universe is something other than baryonic. How many people even know what baryonic means?" comments Walter Stockwell, "Most of the universe is something other than human. If their philosophy already accepts that humans are not the center of the universe, then saying protons and neutrons aren't the center of the universe doesn't seem like much of a stretch to me" (10). Perhaps the only thing a dark matter discovery will give us is some perspective.
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2. Griest, Kim. "The Search for the Dark Matter: WIMPs and MACHOs." Annals of the New York Academy of Sciences. vol 688. 15 June 1993: 390-407.
3. Gribben, John. The Omega Point: The Search for the Missing Mass and the Ultimate Fate of the Universe. New York: Bantam, 1988.
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6. Wilford, John Noble. "Astronomy Crisis Deepens As the Hubble Telescope Finds No Missing Mass." New York Times. 29 Nov. 1994: C1-C13.
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10. Stockwell, Walter K. E-mail interview. 1 Feb. 1995.
11. McIrvin, Matt. "Some Frequently Asked Questions About Black Holes." physics-faq/part2. sci.physics Newsgroup. 5 Dec. 1994.
12. Asker, James R. "'Missing Mass' Enigma Deepens." Aviation Week & Space Technology. 21 Nov. 1994: 31.
13. Falco, Emilio and Nathaniel Cohen. "Gravity Lenses: A Focus on the Cosmic Twins." Astronomy. July 1981: 18-22.
14. Wilford, John Noble. "New Galactic Evidence of Black Holes." New York Times. 12 Jan. 1995: B9.
15. Miyoshi, Makoto., et al. "Evidence for a Black Hole from High rotation Velocities in a Sub-parsec Region of NGC458." Nature. 12 Jan. 1995: 127-129.
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Shall the WIMPs Inherit the Universe?
By Wil Milan
Special for SPACE.com
posted: 01:00 pm ET
28 February 2000
One of the great mysteries in physics why 90 percent of the universe's mass appears to be invisible came one step closer to resolution last week.
A new sensor system that can pinpoint the presence of a theorized form of "dark matter" known as "WIMPs" (Weakly Interacting Massive Particles) has released its first findings though the results seem to raise as many questions as they answer.
Catching WIMPS The new detector was developed by the Cryogenic Dark Matter Search (CDMS), a collaboration of 10 institutions and researchers from around the world. Its advantage is that it can discriminate between "hits" caused by WIMP impacts and other phenomena that cause similar impact effects.
WIMPs may account for much of the "dark matter" in the universe, but because WIMPs do not usually interact with other matter, they slip through the universe undetected. In the last decade new sensors have been built that can detect the presence of WIMPs by detecting their sporadic impacts on other atomic particles, but these sensors have been hampered by many "false hits" due to cosmic rays and other phenomena.
How it works At the heart of the CDMS sensor is a mass of silicon and germanium crystals. The CDMS detector works by sensing the recoil energy when a particle strikes the nucleus of an atom in these crystals, each impact being sensed as ionization and heat. Heat is the result of atoms vibrating, but normally the tiny amount of heat caused by a single tiny impact would make such a small difference in the temperature of the sensor that it would not be detectable.
But that is where the cryogenic feature of the detector comes in: By cooling the crystals almost to absolute zero, the normal atomic vibrations in the material of the sensor are almost completely stopped. With the normal heat vibration virtually stopped, any vibrations caused by a particle impact become much more evident and they can be counted as "hits."
However, the CDMS sensor can not only count hits, it can also tell what kind of "hit" it was. By carefully measuring the amount and type of energy released by each impact, it can tell if the impact was that of common radiation (such as beta and gamma rays) or of a heavy particle such as a WIMP. The sensor does this in two ways:
It measures the level of ionization caused by each impact. Beta and gamma ray impacts cause relatively high levels of ionization, and thus any such impacts can be eliminated as not being WIMP impacts.
In addition, the detector can measure the amount of heat energy released by the impact and therefore tell if it was a heavy-particle impact on an atomic nucleus, which could be due to a WIMP. It's analogous to listening to bullets striking a heavy metal plate: A small-caliber bullet would barely affect the metal plate, while a big bullet would set it ringing like a gong. The CDMS detector can measure the amount of "ringing" caused by each impact (measured as heat from the detector) and thereby tell if the impact was caused by a light impact (such as from a gamma ray) or from a heavy particle such as a WIMP.
Deafening silence And yet, to date the main finding of the detector has been nothing. In its first year of operation the CDMS detector has recorded 13 nuclear impacts -- most or all of which could be explained by things other than WIMPs. However, this finding is itself quite significant because it is at odds with results from an earlier type of WIMP detector that seemed to yield many more hits.
The large discrepancy may seem strange, but even stranger is that both results may be correct. According to Dr. Bernard Sadoulet, one of the researchers involved in the project, "[The] physics for this unknown particle may be different from what we expect, and it is theoretically possible that we are both right!"
WIMPs winning, MACHOs losing The quest to verify WIMP theory recently became more urgent because of the partial demise of its competitor, which is known as MACHO theory. The theory of MACHOs (MAssive Compact Halo Objects) holds that the "missing matter" is not made up of elusive subatomic particles, but by galaxies having a "halo" of plain old everyday matter that happens to be dark and therefore invisible. This would include burned-out dark stars, stray planets and other large, heavy, but dark clumps of common everyday matter. Such objects would be invisible to telescopes, but if they were ubiquitous throughout the universe they could account for the "missing matter."
To confirm the existence of MACHOs, telescopes have been set up to watch for the brief "eclipses" caused by these dark objects moving in front of distant stars. But after more than five years of searching, very few potential MACHOs have turned up, and MACHO researchers recently reported that they now believe that MACHOs cannot account for most of the "missing matter."
The elimination of MACHOs as the primary explanation of the "missing matter" is providing additional impetus for improved WIMP theories and WIMP detectors. Plans are already underway for larger WIMP detectors, while theoreticians race to find new theories that would explain the conflicting results, as well as help devise improved detectors. The potential end result confirmation of the existence of a whole new type of matter that makes up most of the universe would have enormous implications for our understanding of the universe, and thus this is expected to remain a hot research area for years to come.